The optional contribution of the divine command to the validity of normative provisions

Where do ethical claims come from, and why can normative instructions be derived from them? What is the validity of these claims?
Are they an expression of a sense of moral disgust ("You must not murder" = "I feel bad when I see murder") or do they express some ontological truth, and if so, what is it? Where does it come from, and why would I act on it when I could act differently?
Of course, it will not be possible to answer all of these questions in one response.
My central question, arising from the above questions, is: What is the difference between ethical claims based on divine command and those that are not, regarding the validity of normative instructions? In other words, why is God an entity to be obeyed, beyond his definition as such?
The question is a follow-up to another question I asked yesterday, but I was unable to leave a comment on the original question page.